This work is aimed at a careful and accurate study of model membrane systems using electron-spin relaxation techniques with nitroxide and vanadyl probes. We shall use ESR linewidth and lineshape studies as well as ENDOR, ELDOR, and spin echoes. In our initial work we shall concentrate on hydrated dipalmitoyl phosphatidylcholine bilayers and focus on molecular dynamics in the various phases exhibited by this system, which we shall study as a function of temperature, pressure, and sample orientation. The spin-relaxation analyses will be based on the rigorous theories of Freed and coworkers for motional and slow-tumbling effects, which have now been extensively applied to (thermotropic) liquid crystals. While previous ESR studies on model membranes have been interpreted approximately, revealing important properties such as membrane fluidity, segmental motion, polarity probe location, and rates of rotational and translational diffusion, our proposed studies will refine and extend this approximate work and will also reveal, on a microscopic level, types of molecular reorientation, cooperativity at the thermal phase transition, biaxiality, the nature of the molecular forces, effects of local structure, and elastic properties. These studies will be extended to protein-lipid interactions using a well established reconstituted membrane system containing biologically active Ca2 ion ATP-ase. A primary objective will be to elucidate the dependence of enzyme activity on the state of molecular motion of the membrane lipids, including the pH control of Ca2 ion binding.